MECHANISTIC MODELING OF PHOSPHATE PLACEMENT TO MAXIMIZE PHOSPHATE RECOVERY BY SOYBEANS

CLOVIS MANUEL BORKERT, Purdue University

Abstract

Previous research has shown inconsistent results from P fertilizer placement. The objective of this research was to study the effect on the mechanisms governing P uptake when fertilizer is placed in various volume fractions of the soil. Experiments were conducted in pots in a controlled climate chamber. Kinetic parameters for P uptake, root distribution and morphology and soil supply parameters were measured. Increasing rate of P added to the soil increased P concentration in soil solution (C(,l)) and the P diffusion coefficient (D(,e)), while decreasing the buffer power (b). The initial rate of added P increased root length and decreased root radius, while higher rates of P depressed root growth. Rate of P uptake (influx) increased as C(,l) increased, however, yield reached a maximum and then decreased when C(,l) became very high. The effects of P placement on soil and plant parameters were studied by decreasing the soil volume to which the same rate of P per pot was added. Values of soil parameters were determined for both P fertilized and unfertilized soil. Adding P fertilizer to decreasing soil volumes stimulated root growth in the fertilized portion while the root radius became smaller in this zone. The relation between volume fraction of the soil fertilized by P and fraction of the total root length that was in the fertilized soil could be described by (')Y = X('0.70), where X is the proportion of the soil volume treated with P and (')Y is the predicted proportion of total root length in the P treated soil. In a split-root solution experiment decreasing the amount of roots in +P solution increased I(,max) (maximum influx) while K(,m) (Michaelis-Menten constant) and C(,min) (minimal concentration) were not affected. The increase in influx was not large enough to compensate for the decrease in proportion of roots exposed to P, so P uptake per plant and shoot dry weight decreased. Experimental data were used in a mathematical model to predict P uptake for various P distributions in the pot. Predicted P uptake agreed with observed P uptake (r = 0.99**). Predicted P uptake was calculated for a wide range of P rates and placements. These predictions were then used to determine placement giving the maximum P uptake for each rate. For example, the greatest P uptake occurred when 240 mg P kg('-1) was mixed with 12.5% of the soil volume. This procedure could be used to determine the most efficient P placement for other soils. Placement will vary with the reaction of the added phosphate in the soil.

Degree

Ph.D.

Subject Area

Agronomy

COinS